Fix empty set repr

stdlib/internal/types/collections/set.codon

@@ -133,7 +133,7 @@ class Set:
         from internal.types.strbuf import strbuf
         n = self.__len__()
         if n == 0:
-            return "{}"
+            return "set()"
         else:
             buf = strbuf()
             buf.append("{")

test/core/containers.codon

@@ -383,7 +383,7 @@ def test_set():
         pass
 
     assert repr({(1,2)}) == '{(1, 2)}'
-    assert repr(Set[int]()) == '{}'
+    assert repr(Set[int]()) == 'set()'
 test_set()
 
 @test